1. Field of the Invention
The present invention relates to wireless communication systems, and more particularly, to a system and method for initial ranging in wireless communication systems.
2. Related Art
In the field of wireless communication technology, recent demands for high data rates have stimulated intense research activity in multicarrier modulation techniques. Particular interest has been directed to orthogonal frequency-division multiple-access (OFDMA) wireless communication systems, which have become part of the Institute of Electrical and Electronics (IEEE) 802.16 family of standards for wireless metropolitan area networks (WMANs). OFDMA systems allow for the transmission of digital information between a base station (BS) and a plurality of remote user devices, such as cellular telephones, wireless handheld computers, etc. OFDMA systems split the available bandwidth into smaller subchannels composed by a set of orthogonal subcarriers, which are assigned to different users to simultaneously communicate with the BS. The user signals are received at the BS as a series of orthogonal frequency division multiplexing (OFDM) blocks, from which information data are extracted by the BS. Unfortunately, OFDMA systems are extremely sensitive to timing errors and carrier frequency offsets (CFOs) that may occur between uplink signals and the local references at the BS. Timing errors give rise to interblock interference (IBI), while inaccurate compensation of the CFOs destroys orthogonality among subcarriers and produces interchannel interference (ICI) as well as multiple access interference (MAI).
In an attempt to alleviate these drawbacks of OFDMA systems, the IEEE 802.16 standards specify a synchronization procedure called Initial Ranging (IR), wherein users that intend to establish a link with the BS adjust their transmission parameters so that uplink signals arrive at the BS synchronously and with approximately the same power level. In its basic form, the IR process develops through the following steps. First, a remote user computes frequency and timing estimates on the basis of a downlink control channel. The estimated parameters are then used in the uplink phase, during which each remote user transmits a randomly chosen code over a ranging time-slot that comprises a specified number of adjacent OFDM blocks. However, as a consequence of the different users' positions within the cell, uplink signals arrive at the BS at different time instants. Furthermore, since the ranging time-slot is randomly selected, several users may collide over a same ranging subchannel. After separating colliding codes and extracting timing and power information, the BS broadcasts a response message indicating the detected codes and giving the necessary instructions for timing and power adjustment.
The main functions of the BS during the ranging process may be classified as multi-user code detection, and multi-user timing and power estimation. In existing IR techniques, a long pseudo-noise (PN) code is transmitted by each RSS over all available ranging subcarriers. Code detection and timing recovery is then accomplished on the basis of suitable correlations computed in either the frequency or time domains. These approaches require huge computational complexity since each correlation must be evaluated for each possible ranging code and hypothesized timing offset. Moreover, separation of users is achieved by exploiting only the correlation properties of the employed code set. In the presence of multipath distortions, however, ranging subcarriers are subject to different attenuations and phase shifts, thereby leading to a loss of the code orthogonality. This gives rise to MAI, which severely degrades the system performance.
Alternative solutions involve replacing the PN ranging codes with a set of modified generalized chirp-like (GCL) sequences and mitigating the effects of channel distortion through differential detection of the ranging signals. Unfortunately, this approach is still plagued by significant MAI and, furthermore, it cannot be applied to interleaved OFDMA systems since ranging subcarriers must be assigned adjacently. Some attempts at reducing the system complexity rely on the decomposition of the multi-parameter estimation problem into a number of successive steps. However, they are only suitable for flat channels and fail in the presence of multipath distortions. Still another approach involves a reduction of the computational burden, but this is achieved at the price of an increased overhead using a dedicated common ranging code.
A common drawback, among others, of existing IR methods is their poor performance in the presence of frequency selective channels. For example, in one existing method a signal design is proposed which is robust to multipath distortions, wherein ranging signals are divided into several groups and each group is transmitted over an exclusive set of subcarriers with a specific timing delay. This approach leads to a significant reduction of MAI as signals of different groups are perfectly separable in either the frequency or time domain. Additionally, in existing approaches, phase rotations induced by residual CFOs between the uplink signals and the BS frequency scale are ignored even though such rotations may become significant over ranging periods spanning several adjacent blocks. In such cases, the received ranging signals are no longer orthogonal and CFO compensation is necessary to avoid a serious degradation of the system performance. Unfortunately, frequency recovery schemes for OFDMA systems are normally derived under the assumption that uplink signals are transmitted over disjoint subcarriers and, as a result, cannot be applied to a scenario where multiple codes share the same subchannel.
Accordingly, what would be desirable, but has not yet been provided, is a system and method for initial ranging in wireless communication systems, which addresses the foregoing shortcomings of existing initial ranging approaches.